This invention relates to an antenna arrangement system for use in a mobile communication system, such as a mobile telephone system, which has a service area assigned thereto.
In a conventional mobile communication system of the type described, a cellular system has been adopted 7hich divides a service area into a plurality of cells which may approximately be represented by regular hexagonal configurations, respectively, and in which a plurality of radio base stations are located at centers of the cells so as to cover the respective cells with the radio base stations. In such a cellular system, consideration is made about effective use of radio frequency channel. For this purpose, the same radio frequency channel is repeatedly assigned to or reused in the radio base stations which are spaced apart from one another so that no co-channel interference occurs among the radio base stations.
When the same frequency channel is repeatedly used or reused among the radio base stations, the radio base stations are divided into object base and interference base stations which suffer from the co-channel interference and bring about the co-channel interference into the object base stations, respectively.
Herein, let the radio base stations have omnidirectional antennae located at the centers of the cells, respectively, and be clustered into a plurality of groups each of which includes a prescribed number of the radio base stations to define the prescribed number of the cells. This shows that the prescribed number defines a cluster size or a repetition number of the cells in each cluster. In this event, an amount of the co-channel interference depends only on a distance between the object and the interference base stations. Under the circumstances, it is necessary to separate the object and the interference base stations from one another as long as possible in order to minimize the amount of co-channel interference with the cluster size kept invariable.
Specifically, maximum distances between the object and the interference base stations are obtained when six of the interference base stations are equidistantly located around the object base station, provided that each cell is approximated by the hexagonal configuration.
In an alternative antenna arrangement system, each of the cells is divided into a plurality of sectors with a single radio base station located at the center of each cell. This antenna arrangement system will be referred to as a sector type of the antenna arrangement system. In this case, each of the radio base stations has a plurality of directional antennae which can cover the respective sectors. The same or identical frequency channel is reused at every cluster size to provide a reuse pattern of the same frequency channel.
With this structure, it is possible to reduce a cell size as compared with the case where the omnidirectional antenna is located in each cell, as mentioned above. In addition, the same frequency channel may be arranged in the service area with a reduced distance between the object and the interference base stations by the use of directivity of the antennae arranged in each sector of the cells. This means that each frequency channel is effectively utilized in the sector type of the antenna arrangement system.
The reuse patterns in the sector type of the antenna arrangement system are classified into a parallel beam pattern system and a back to back beam pattern system. Description is made about the parallel beam pattern system in a paper which is contributed by J. E. Stjernvall to Nordic Seminar on Digital Land Mobile Radiocommunication held on Feb. 5-7, 1985, at Eapoo, Finland and which is entitled "CALCULATION OF CAPACITY AND CO-CHANNEL INTERFERENCE IN A CELLULAR SYSTEM". In such a parallel beam pattern system, the cells are approximated by regular hexagons or regular hexagonal configurations and groupled into clusters each of which has a cluster size composed of a predetermined number of the cells which may be equal to three, nine, twelve, twenty-one, or the like. For brevity of description, it is assumed that each cluster is composed of three cells and that first through third ones of the frequency channels are allocated to each cluster. In this event, the first frequency channels are directed through first predetermined sectors in a first direction identical with one another in the respective clusters. Similarly, the second frequency channels are directed through second predetermined sectors towards a second direction which is identical with one another in each of the clusters and which is different from the first direction. This applies to the third frequency channels. At any rate, the first through the third frequency channels are oriented in the same directions, respectively. This shows that each of the first through the third frequency channels is parallel to one another in the respective clusters. It is to be noted that sector distances among the first or the second sectors of the respective clusters are variable.
With this parallel beam pattern system, strong co-channel interferences take place between the same frequency channels in dependency upon a minimum one of the sector distances.
Alternatively, the back to back beam pattern system is disclosed by G. Falciasecca et al in a paper which is contributed to Second Nordic Seminar (October, 1986) and which is entitled "OPTIMUM USE OF RADIO SPECTRUM IN MOBILE RADIO SYSTEMS ACCORDING TO TRAFFIC PARAMETERS". According to the back to back beam pattern system, it is possible to reduce distances among object and interference base stations as compared with the parallel beam pattern system. However, consideration is made only about coexistence of the omnidirectional antennae and the directional antennae. In other words, such a back to back beam pattern system is locally adopted in a part of the service area wherein traffic is extremely congested. As a result, no consideration is made at all about adopting the back to back beam pattern system over a whole of the service area.
Thus, co-channel interference among the same frequency channels can not be sufficiently removed because directivity of antennae of each base station is effectively used in either the parallel beam pattern system or the back to back beam pattern system.